Added get_num_groups

The get_num_groups function was missing for r600g. I did the same
thing as the other workitem functions.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Aaron Watry <awatry@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@187059 91177308-0d34-0410-b5e6-96231b3b80d8